All good information and now need to digest it.

So, what is the thickness of the floor? Does the elevation of a level start at the upper surface of a floor, or does the floor 'fill up' the level from its base elevation?

Floors are added to the height setting of the level. In other words, if you set the height of the level at 8' and the floor thickness at 4" , the total height of the level is 8'4", with the 4" at sub-level.

Ok, I think I got that.

Are walls drawn from the elevation of their level, or on top of the floor?

Walls are built on top of the floor.

OK.

What about ceilings? Do they have any thickness?

Ceilings are one sided so there is no thickness to them. Just a one sided poly.

Ok, that's clear.

E.g. I have an outside door in a wall. From where is its elevation calculated? From the bottom of that level, or from the floor above that?

Door and window heights are set from the top of the floor. The floor is actuall below level.

I tried some tests. I had the walls set and placed a table in the middle. I thought increasing the floor height would raise the table, but it didn't, which I can now understand. But there seemed to be no change at all, yet the table and walls of that level should have moved up with increasing floor thickness, which I didn't notice. I'll try it again with a (slightly) lower level also displayed as then I should see the one level move higher with increasing floor thickness, compared to the other, slightly lower level, yes?

So if everything sits on top of the floor, what's the point of being able to set the thickness of the floor.

For the ground floor of a house with e.g. everything built on a concrete base say 150mm thick. I could set the elevation of that level to 0 and the floor thickness to .15m, but can I see the edge of the floor? And how can I create outside the house since everything will be 150mm above the bottom of that level? Or is this a case for another level at the same elevation, but with 0 floor thickness?

That also leads on to doors and windows. When specifying the width, is that the width of the actual door/window, or the opening in the wall?

Do you mean the depth/thickness of the doors and windows?
The default doors are 3' wide, 11 5/8" thick and 7' tall.

I meant the width of the door/window as we would think of it, but in SH3D terms I guess that's the length. I think it corresponds to the length/width of the opening in the wall, inside which is the frame and the actual door or window, but confirmation of that would help.